1. Field of the Invention
The present invention relates to an image forming apparatus such as an electrophotographic printer apparatus having, e.g., a spare paper feed function. In this specification, the spare paper feed function is defined as a function of feeding a recording sheet or the like from a paper feed cassette or the like to a predetermined position prior to a print request (or an end of one-page image development), and feeding the paper sheet from the predetermined position to a recording unit upon reception of a print request.
2. Related Background Art
A conventional image forming apparatus such as a laser printer performs predetermined image formation in accordance with a recording request signal (PRINT), a vertical sync request signal (VSREQ), a vertical sync signal (VSYNC), and an image signal (VDO). FIG. 1 shows a schematic arrangement of a laser printer, FIG. 2 is a block diagram of its controller, and FIG. 3 is a timing chart showing a recording operation of the laser printer.
A printer controller 100 develops an image on the basis of code data from an external host computer (not shown) or the like and outputs the signal PRINT when bit map image data of one page is obtained. A paper feed roller 49 starts to feed a sheet to a resist roller 50 in accordance with the signal PRINT from the printer controller 100. A printer engine control unit 101 supplies the signal VSREQ to the printer controller 100 at a predetermined timing when a resist sensor 51 detects a leading end of the recording medium. The printer controller 100 immediately outputs the signal VSYNC and causes a semiconductor laser (not shown) to emit laser light 46, thereby starting exposure. In synchronism with the exposure operation, the resist roller 50 is driven, and the recording medium is fed toward a photosensitive drum 41. A developing unit 43 develops a latent image formed on the photosensitive drum 41 to form a toner image. The toner image formed on the photosensitive drum 41 is transferred to a recording medium by a transfer roller 44 and is fixed on the recording medium by a fixing roller 48. A charge roller 42 uniformly charges the photosensitive drum 41. A cleaner 45 removes a toner left on the photosensitive drum 41. A polygonal mirror 47 scans the laser light 46 in a direction perpendicular to the drawing surface.
In the conventional arrangement, however, a series of recording operations are performed after the signal PRINT becomes true or ON. Even if a command signal representing resolution switching is received during the recording operation, processing represented by the command signal cannot be performed on the recording medium fed in response to the signal PRINT. That is, since the start of recording is designated by only the signal PRINT, the command signal received upon reception of the signal PRINT is neglected or becomes effective for printing the next page. The command effective for the already fed recording medium upon reception of the signal PRINT represents cancellation of printing which is realized by not holding the signal PRINT until the trailing end of the signal VSYNC.
In the conventional technique, as described above, a recording mode of the already fed recording medium by the user cannot be changed upon start of the paper feed operation. A recording mode must be determined prior to generation of the signal PRINT. In addition, even in the print cancellation represented by the command effective upon generation of the signal PRINT, the print enable state cannot be effective unless the fed recording medium is discharged. A throughput is undesirably decreased, and the recording medium is wastefully discharged.
This problem typically occurs in an image forming apparatus having a spare paper feed function proposed in U.S. patent application Ser. No. 558,322 filed by the present applicant due to the following reason. For example, even if a command for selecting one of a plurality of paper feed entrances is output after the start of spare paper feeding, it is neglected or becomes effective in feeding of the next sheet. That is, the user must enter a command for determining a recording mode prior to the start of spare paper feeding to the image forming apparatus.
The image forming apparatus having the spare paper feed function will be described below. In an image forming apparatus of this type, for example, the printer controller 100 outputs a spare paper feed request signal for informing spare paper feeding to the printer engine control unit 101 prior to the end of image development of one page. When the printer engine control unit 101 receives the spare paper feed request signal, the control unit 101 controls a paper feed control unit 105 to stop, e.g., a recording medium in a state wherein it abuts against the resist roller 50. The printer engine control unit 101 waits for a print request signal from the printer controller 100. The following operations are the same as described above.
An abnormal event handle unit 111 monitors a state of the inside of the printer and alarms an abnormal event to the printer engine control unit 101 when the abnormal event which causes failure of normal print processing occurs.
In the conventional arrangement, the abnormal event handle unit 111 does not distinguish abnormal event handling for an abnormal event occurring during printing while a print request signal is received from the printer controller 100 from abnormal even handling for an abnormal event during paper feeding upon reception of a spare paper feed request. Therefore, the abnormal event handle unit 111 performs single processing.
Conventional processing performed upon occurrence of an abnormal event for removing a paper feed cassette 13 (FIG. 1) will be described with reference to flow charts in FIGS. 4 to 6.
Upon reception of a print request from the printer controller 100, the paper sheet is fed to the resist roller 50 by driving of the paper feed roller. At this time, a vertical sync request signal is output to the printer controller 100. Upon reception of the vertical sync signal from the printer controller 100, an image data output instruction is output to an image control unit 103. At the same time, a timer for measuring a paper feed timing of the next page is started. When the time set in the timer has elapsed and if a print request for the next page is not present, the in-print sheet is discharged and the feed system is stopped. However, if the print request is present, a paper feed instruction for the next page is output. At this time, although the presence/absence of the paper feed cassette is always checked, the paper sheet is not fed and stopped upon reception of the print request in abnormal event handling. That is, assume that a spare paper is fed upon reception of a spare paper feed request, that the paper sheet is fed to a predetermined position, i.e., the resist position, and that the apparatus waits for a print request. Under these conditions, when the cassette is temporarily removed and a print request is received after the cassette is then mounted again, the normal vertical sync request is output because the cassette is present at this time. Upon reception of the vertical sync signal, an image data output instruction is output.
In the conventional arrangement described above, no-paper processing upon reception of a print request is the same as no-paper processing upon reception of a spare paper feed request. For this reason, even if the paper feed cassette is removed in a print request wait state once a spare paper feed operation is performed and when the paper feed cassette is mounted again, the vertical sync request is output in response to a print request even under the conditions wherein the spare paper sheet is removed and an abnormal event occurs. Upon reception of the vertical sync signal, image data output is undesirably started. The transfer roller is directly transferred with a toner image. As a result, the lower surface of the sheet is contaminated. In addition, for example, even if the trailing end of the recording medium is torn, it is undesirably output as a normal printed sheet.
The sheet in spare paper feeding receives a deformation force by a loop, a nip of the resist roller 50, and a pressure of the paper feed roller 49.
In recent years, a time required for image development tends to be prolonged along with complication of a recording image. In addition, since the recording medium is exemplified by a thin sheet such as normal paper, intermediate sensitized paper, an OHP film, synthetic paper, or an envelope, when the thin sheet is left in the printer in this state for a long period of time, it is locally warped (curled).
The curled portion is not properly brought into contact with the photosensitive drum during image transfer, thus causing a transfer error. More specifically, a white omission occurs because the toner is not locally transferred to the sheet. In addition, since the image is transferred to the sheet while it is not properly brought into contact, image blurring occurs, thereby degrading recording image quality.
The following problem occurs in an image forming apparatus having a spare paper feed function when one of a plurality of paper feed means is designated to feed a recording medium.
FIG. 7 shows a schematic arrangement of a laser beam printer having a plurality of paper feed means.
An electrophotographic photosensitive drum 1 is rotated at a predetermined peripheral velocity (process speed) clockwise, i.e., a direction indicated by an arrow.
The photosensitive drum 1 is uniformly charged to a predetermined polarity at a predetermined potential by a charge roller serving as a charging means during rotation.
The charged surface of the photosensitive drum 1 receives laser beam scanning exposure light L (image write) by a laser scanner 5 which includes a laser oscillator, a polygonal scanner, and an optical element and which outputs a laser beam modulated in correspondence with a target time-series electrical digital pixels signal input from a host apparatus (not shown) such as a computer or a wordprocessor to a controller (not shown). A latent image corresponding to the target image information is formed on the circumferential surface of the photosensitive drum 1.
The latent image on the drum surface is visualized by a developing unit 3 into a toner image. The toner image reaches a transfer portion B as an opposite portion between a transfer roller 4 and the drum 1 during rotation of the drum 1. The toner image is continuously transferred from the photosensitive drum 1 to a recording sheet fed from a sheet material feed unit (to be described later) to the transfer portion B at a predetermined timing determined by a resist roller 9.
The recording sheet to which the toner image is transferred is separated from the surface of the photosensitive drum 1 and is fed to a fixer 12. The image is fixed on the sheet, and the sheet as an image formation product (print) S.sub.4 is discharged to a paper discharge tray 20 by a paper discharge roller 19.
The surface of the drum 1 which passed the transfer portion B is subjected to removal or cleaning of a residual pile such as a residual toner by a cleaner 6 and is repeatedly used for image formation.
The sheet material feed units are constituted by the following three paper feed means:
a. a cassette type middle automatic paper feed unit 31 (first paper feed means) for feeding each sheet;
b. a cassette type lower automatic paper feed unit 32 (second paper feed means) for feeding each sheet; and
c. a manual paper feed type upper paper feed unit 33 (third paper feed means).
When a printer engine (not shown) receives a print signal from a controller, the printer engine operates a preselected one of the three paper feed means 31 to 33 to feed a sheet. More specifically,
(1) When the first paper feed unit 31 is selected and designated, a paper feed roller 7.sub.1 of the first paper feed means is rotated and driven to feed one of recording sheets S.sub.1 stacked in the paper feed cassette of the first paper feed means. The fed paper sheet passes through a separate sheet path P.sub.1 of the first paper feed means 31 and enters into a common sheet path P.sub.4 connected to the transfer portion B.
The resist roller 9 is disposed in the common sheet path P.sub.4. A resist sensor 10 is arranged in front of the resist roller 9 to detect the leading end of the recording sheet as a fed sheet material.
The leading end of the recording sheet S.sub.1 entering into the common sheet path P.sub.4 is detected by the resist sensor 10. At this moment, the leading end abuts against the nip portion of the roller pair as the resist roller 9 which is stopped.
Feeding of the recording sheet S.sub.1 by the paper feed roller 7.sub.1 is performed as follows. When the leading end abuts against the resist roller 9 after the leading end of the recording sheet is detected by the resist sensor 10, the sheet is temporarily stopped when a period of time for forming a predetermined loop of the recording sheet has elapsed. A vertical sync request signal is transmitted to the controller, and reception of the vertical sync signal is awaited.
Upon formation of the loop of the fed recording sheet, the leading end of the recording sheet entirely abuts against the nip line of the roller pair as the resist roller 9 to eliminate ramp of the recording sheet and perform matching of the resist portion. Upon reception of the vertical sync signal from the controller after the paper feeding is temporarily stopped, image writing on the photosensitive drum 1 is allowed, and at the same time, the resist roller 9 is rotated and driven.
Upon rotation and driving of the resist roller 9, feeding of the recording sheet S.sub.1 temporarily stopped at the roller 9 is started again. The sheet passes through the common sheet path P.sub.4 and reaches the transfer portion B. When the leading end of the recording sheet reaches the transfer portion B, the leading end of the toner image formed on the surface of the photosensitive drum 1 also reaches the transfer portion B, so that the toner image surface of the photosensitive drum 1 matches with the recording sheet surface, and image transfer is performed.
(2) When the second paper feed means 32 is selected and designated, a paper feed roller 7.sub.2 of the second paper feed means is rotated and driven to feed one recording sheet S.sub.2 from the paper feed cassette of the second paper feed means 32. The fed paper sheet passes through a separate sheet path P.sub.2 of the second paper feed means 32 and enters into the common sheet path P.sub.4 connected to the transfer portion B.
The same paper feed operations as in the recording sheet S.sub.1 of (1) are performed, and the sheet is synchronously fed to the transfer portion B by the resist roller 9.
(3) When the third paper feed means 33 is selected and designated, a paper feed roller 7.sub.3 is rotated and driven to feed a recording sheet S.sub.3 placed on a manual feed tray of the third paper feed means 33. The sheet passes through a separate path P.sub.3 of the third paper feed means 33 and enters into the common sheet path P.sub.4 connected to the transfer portion B.
The same paper feed operations as in the paper sheet S.sub.1 of (1) are performed, and the sheet is synchronously fed to the transfer portion B by the resist roller 9.
A point A is defined as a merge point between the separate sheet paths P.sub.1 to P.sub.3 of the first to third paper fed means 31 to 33 and the common sheet path P.sub.4. In the third paper feed means 33, a sensor 8 is arranged to detect the presence/absence of the recording sheet S.sub.3 set on the manual feed tray.
In the above printer, for example, if a sheet path length from the paper feed position (i.e., a position of the leading end of the recording sheet in each cassette) to the resist roller 9 is large, printing of the next page must be started during write access of the previous image to obtain a maximum throughput. For this reason, the recording sheet S.sub.2 must be fed from the paper feed position to the predetermined position in advance (spare sheet feeding).
In the conventional apparatus described above, however, when the plurality of paper feed means are provided, and when paper feeding from another paper feed means except for the currently selected paper feed means is designated from the controller while the paper is subjected to spare sheet feeding from the preselected paper feed means and is stopped, the sheet in spare paper feeding at the merge point of the separate sheet paths of the respective paper feed means is present and blocks the feed path, thereby disabling paper feeding from a paper feed means except for the preselected paper feed means.